At least 45 people, including a dozen minors, were killed after a bus caught fire south of the Bulgarian capital early Tuesday morning, officials said.
A cause has yet to be determined but officials believe a fire broke out on board and the bus crashed into guardrails.
There were no other vehicles involved in the accident, which occurred around 2:00 am (2400 GMT) on a highway about 40 kilometres (26 miles) from Sofia, near the village of Bosnek.
"Of the victims ... 12 in total were under the age of 18," national police chief Stanimir Stanev said.
He told bTV television that 45 of the 52 people on the bus were killed.
Nikolay Nikolov, head of the Fire Safety and Civil Protection department at the interior ministry, told public broadcaster BNT that "seven passengers survived".
They were taken to a hospital in the capital with serious burns, he added.
According to bTV, the bus was travelling from Turkey's main city of Istanbul to Skopje in North Macedonia.
North Macedonian Prime Minister Zoran Zaev said the victims were probably fellow Macedonians.
"We don't know if all the victims are from North Macedonia, but we assume so because the bus is registered in the country," he said in an interview with Nova TV.
But police chief Stanev said while the two drivers of the bus were Macedonian, the passengers were Albanian.
"Initial information shows that 52 people were travelling in the bus, including two drivers with Macedonian nationality and 50 passengers with Albanian nationality," he said.
Bulgaria's interim Prime Minister Stefan Yanev and Interior Minister Boyko Rashkov rushed to the site of the crash Tuesday morning, while local media said that the North Macedonian and Albanian premiers were also on their way.
"It's a terrifying picture in there. I haven't seen anything like that before," Rashkov told journalists at the site.
"Nobody can say for certain how many are there and who they were. The bodies are badly burned and have to be identified one by one," he added.
Bulgaria has a history of deadly bus accidents. Seventeen Bulgarian tourists died in 2018 when their bus skidded on a wet road and overturned.
A total of 628 people died in road accidents in 2019 and 463 in 2020 in the country of 6.9 million people, according to official data. The accidents were often attributed to poor road conditions, outdated cars and speeding.